The present invention is directed to a capacitive probe for use in dimensional gauging. More specifically, the invention is directed to a non-contact, omni-directional probe for use with a coordinate measurement machine (CMM).
Computerized CMMs have become standard equipment in machine shops, QA laboratories, and precision engineering labs. The CMMs guide a sensing probe in a coordinate measurement system to obtain dimensional information from a workpiece.
Several different sensor probe types are available. In systems with mechanical contact type sensor probes, the sensor probe is guided into contact with the workpiece until a switch in the probe is opened or closed. The CMM computer acquires the dimensional information upon activation of the switch.
Such mechanical contact probes have several disadvantages. Mechanical contact probe typically require contact forces in excess of a tenth of a newton thus rendering them useless in, for example, gauging silicon structures which cannot be touched in a non-destructive manner.
Non-contact probes may be used in lieu of the mechanical contact probes described above. One type of non-contact probe is described in McRae, "Using Capacitive Sensing for Noncontact Dimensional Gauging", Sensors, pp. 13-20, October 1988. The article describes the use of a flat plate sensor that forms a parallel plate capacitor when placed adjacent a conductive target plate (e.g. workpiece). The capacitance of the parallel plate capacitor is inversely proportional to the distance between the sensor and the target plate. Thus,
C=K/d where PA1 C is the capacitance, PA1 K is a constant, and PA1 d is the distance between the sensor and target plate.
Parallel plate capacitive probes have several disadvantages. They must be precisely aligned in parallel with the target plate so that the capacitive plate area remains relatively constant. Additionally, the unidirectional character of the parallel plate capacitance only allows positional measurement along a single coordinate axis that is normal to the flat probe plate.
It is therefore an object of the present invention to provide a non-contact, omni-directional capacitive probe for use in dimensional gauging.